In a prior art solar cell, an electrode is produced by directly plating a low resistance metal on a surface of solar cell using vacuum evaporation or sputtering or by screen printing a conductive resin on the surface of solar cell and baking it.
FIG. 10 shows a prior art electrode production method using vacuum evaporation. In FIG. 10, reference numeral 1 designates a solar cell having a surface 4. Reference numeral 20 designates a metal mask. Reference numeral 21 designates a low resistance metal, reference numeral 22 designates an electron gun, and reference numeral 23 designates an electron beam.
In this method, the metal mask 20 having an aperture is disposed on the surface 4 of solar cell 1, and the low resistance metal 21 is heated and evaporated by irradiation with the electron beam 23, whereby an electrode comprising a metal thin film is deposited on the surface 4 of solar cell 1 in a high vacuum.
FIG. 11 shows another prior art electrode production method using screen printing. In this method, a conductive resin 25 is applied on the surface 4 of solar cell 1 through an aperture of screen mask 26 using printing means 24, and thereafter, the conductive resin 25 is baked, thereby producing an electrode. In this method, an electrode of larger film thickness is obtained than that obtained by the above-described vacuum evaporation method or sputtering method.
FIG. 12(a) shows a plan view of a solar cell having a conventional electrode structure. FIG. 12(b) shows a cross-section along line XIIb--XIIb of FIG. 12(a). In these figures, an electrode pattern 27 comprising a metal film is disposed on the surface 4 of solar cell 1 by the above-described method. A ribbon of conductive material 28 for transmitting the generated current outside the solar cell 1 is adhered to a predetermined portion of metal film 27 by solder 29.
The device will operate as follows.
The current generated in the solar cell 1 flows into the electrode 27 comprising a metal film via the surface of solar cell 1. Then, the generated current flows through the electrode 27 to the ribbon of conductive material 28 for transmitting the current outside the solar cell which is fixed to a predetermined portion of electrode 27 by solder 29 or a conductive resin, and the generated electric power is supplied to an external element.
In the path of the generated current, power loss arises due to series resistances of the surface and the electrode. In order to minimize the power loss, the electrode pattern is determined by selecting various parameters such as sheet resistance, contact resistance between the surface and the electrode, width, thickness, and number of the electrodes, resistivity of electrode material, and current density generated by the solar cell. Then, the electrode pattern 27 is produced using the metal mask 20 or the screen mask 26, and an electrode is produced on the surface 4 of solar cell 1 using the above-described production methods.
Since the electrode of the prior art solar cell is produced by using evaporation of metal or printing of a conductive resin, the thickness of the electrode is restricted from about several microns to about several tens of microns according to the respective method. Therefore, in order to minimize the power loss, the width of the electrode needs to be broad or the number of electrodes needs to be increased. For example, when the electrode is produced by evaporation of silver to a thickness of 5 microns (resistivity .rho.=1.6.times.10.sup.-6 .OMEGA..multidot.cm), in order to obtain a resistance approximately equal to that of copper wire of 50 microns diameter, the width has to be about 370 microns. When the electrode is produced by screen printing of a conductive resin including Ag to a thickness of 50 microns (resistivity .rho.=5.times.10.sup.-5 .OMEGA..multidot.cm), the width thereof has to be about 1160 microns. As a result, the area of the electrode on the surface of solar cell becomes large, reducing the effective light incidence area of the solar cell and the generated current is reduced. Furthermore, the power loss at these electrodes is undesirably high.